|Fire modeling: An introduction for attorneys
|...by Dr. Vytenis Babrauskas, Fire
Science and Technology Inc.
|Fire modeling is something which is often found to be mysterious by
attorneys. Yet, understanding what it is, what it can do, and what it cannot do
can be vital to successful development of some types of fire cases. The purpose
of this note is to present the basic ideas so that they are understandable by
the non-scientist. Thus, the information should be of value also to fire
investigators, claims adjusters, and other individuals involved with fire
losses. Most of them are not aware of either the strengths or the limitations of
the fire modeling. Thus, in this note the objective is to explain the process in
simple terms, so that a clear picture will emerge how fire modeling can and
cannot be used.
|What is a model?
|Before we can discuss fire models, we must explain what a scientist means by
'model.' The meaning of this crucial term is essential to understand. A model of
anything is, simply, a systematic representation of that thing. Thus, for
example, we can have
The above three examples are probably the main 'representations' which are
used by scientists. A thought model is simply a proposed schema explaining how
something works. Scale models are often used in structural engineering, fluid
dynamics, and have occasionally been used in fire science. Model trains are
familiar to all. A scale model in scientific work is simply a reduced-size
object on which certain measurements will be made. The category which we want to
discuss in this Note is the last type, the mathematical model. In general, a
mathematical model will be a series of equations which describe a certain
process. If the equations are simple enough, they can be solved on the hand
calculator. More commonly, the equations are not so simple. Consequently, a
computer is required for their solution. Thus, in the fire field, we would speak
of "computer fire models." Nowadays, when one speaks of a "fire model," it is
usually understood that one is referring to a "computer fire model." This is
unnecessarily restrictive, however, and other types of models (such as scale
models) remain legitimate scientific forms of model.
- thought models (or conceptual models)
- scale models, and
- mathematical models.
A "computer fire model" is normally realized as a computer program.
This again, is most common, but not necessarily always true. A computer fire
model, for example, could be realized as only a flowchart. From the above, one
can understand why fire modeling is often taken to mean "use of computer
programs for predicting fire," although this would be too restrictive a
|What do fire models do?
|By now, fire modeling has been in use for more than two decades. This
author's computer program COMPF was released in 1975  and was the first
computer program for predicting room fires to be developed in the U.S. Research
in several other countries, however, goes back further. During the subsequent
two decades, tremendous progress was made in the field. Today, many persons who
have only a limited knowledge of fire science have already had a slight exposure
to fire modeling. From this, they are apt to conclude that fire modeling is
something which allows scientists/engineers to 'wave a magic wand' and to
calculate the history of a fire just by working at their computer. On rare
occasions, this can be true. But normally, the situation is not so
The function of COMPF was to predict the fire history within a single room.
The history was represented only after the time of 'flashover' within the room.
Flashover is the point in a fire (it does not occur in all fires) when
the room "fills with flames." The hazard greatly increases from that point on.
Nowadays, various other types of computer fire models are also available to the
scientist. What kind of fire characteristics, then, can a fire model predict?
The list is limited only by the ingenuity of scientists, but we can cite
characteristics which are already routinely being computed:
It can be noted that this list is weighted towards fluid mechanics and
related themes. This is not surprising, since a majority of the researchers
creating fire models have been fluid mechanics specialists. Models also exist
for certain human behavior aspects (e.g., exiting through corridors and stairs)
although these have so far been very little used for practical problem solving;
thus their validity is generally unknown.
- gas and surface temperatures
- flow rates of gas through openings
- heat fluxes impinging on surfaces
- smoke obscuration
- production of certain toxic gas species
- strength reduction and structural failure of building elements
- activation times for sprinklers and detectors
It should be noted that certain characteristics are usually not being
computed. These include:
A list of other fire characteristics that we cannot yet routinely predict has
recently been publicized . The three characteristics above are three
exceedingly important aspects of fire, indeed heat release rate (HRR) has been
referred to as the single most important variable in describing fire hazard .
Likewise, there will not be a fire without ignition and, in most cases, flame
spread is also an essential trait of fire. The way that today's fire models
normally solve a problem is by being given the HRR as input. The flame spread
aspects are usually not made explicit. The most important role of flame spread
is to progressively involve greater areas in burning, that is, to cause a growth
of HRR. Thus, if we have a HRR versus time curve, the flame spread issue has
already been solved. The initial ignition is, simply, assumed to have taken
place, so no computation is made there either.
- the ignitability of objects from small flames
- the spread of fire over surfaces
- the actual 'size' of the fire, that is, its heat release rate
To make a computation using one of our state-of-the-art models, such as
HAZARD , then requires that the modeler supply a HRR curve as input. In some
cases, the HRR curve may already have been published in the literature for a
'similar' burning object. Compendia of data are available which present some
useful, non-proprietary data . However, the variety of items which can burn
is essentially infinite, while the amount of publicly available data is quite
The situation is even more complicated when one realizes that more than one
item can burn. Methods have been suggested for estimating second-item
involvement . However, under most conditions, such procedures entail a great
deal of uncertainty. This can be due to: (a) irregular geometry of the item in
question; (b) not well enough studied ignition response of the item; (c)
inadequately detailed knowledge of local heat fluxes, etc. When one contemplates
the uncertainties then associated with estimating the ignition for the third,
fourth, etc. item, it becomes clear that the ignition sequence of a roomful of
diverse items cannot be predicted with a reasonable degree of confidence.
|The solution to the above difficulty is actually straightforward: when data
are not available, run a fire test. Model development is a difficult, specialist
task. Thus, one cannot expect to say "improve the models," since progress could
hardly be made on a time schedule to suit fire litigation needs, even if the
resources were available. What is possible to do on relatively short notice is
to organize fire tests.
Fire tests have their limitations, too. The largest fire that can be
conducted indoors in a laboratory, under controlled and instrumented conditions
is about 20 megawatts. Physically, this corresponds to one room or a couple of
smallish rooms joined together. Fire models are much less restricted in that
respect. They are available for computing multi-story, multi-room arrangements,
and the rooms do not have to be small enough to fit under a laboratory's exhaust
Thus, the practical solution is to combine fire modeling with fire testing.
Normally, the objects, walls, etc. associated with ignition and early fire
growth are directly reconstructed in the laboratory by procuring exemplars and
creating what is normally termed a sectional full-scale mockup.
Full-scale denotes that real appliances are used, real wall thicknesses are
employed, etc. Sectional denotes that only a slice out of the building is
constructed in the laboratory and not the whole fire environment.
The presumed or alleged ignition sequence is then started in the laboratory
test and measurements are taken of HRR, smoke production, temperatures, heat
fluxes, and other fire variables. Fire modeling is then used to take the
laboratory data of the initial fire stages as an input and to compute the
subsequent stages of fire development. Thus, fire modeling can be viewed as a
direct extension of fire testing, or vice versa.
The confidence in the results produced by the fire model is normally greater
for the intermediate stages of the fire than for the late stages. During the
late stages of fire, a number of additional events can happen. These include
burn-through of partitions, collapse of beams, collapse of occupant goods (e.g.,
rack storage) and similar. Also, it may be expected that firefighting will make
some difference on the outcome of the fire, and this may not be reasonable to
try to predict mathematically. Models do exist which can allow the prediction of
the collapse of structural members, but these require input data which may often
|Tests vs. demonstrations
|It is important to distinguish between a field demonstration and a
large-scale laboratory test. Both involve setting up of an environment intended
to recreate the scene of the fire origin. Both can be used to produce videos for
jury viewing. However, a field demonstration does not collect HRR nor other fire
data which could usefully serve as input to a fire model. Thus, demonstrations
can only be used for video purposes.
The advantage of a demonstration is that it can be conducted in every town
and city. A laboratory test, by contrast, requires use of a fire testing
laboratory, and there are only a handful of such facilities in the country.
The costs, however, are not necessarily much lower for a
demonstration. The bulk of the cost is normally associated with procuring
exemplars, constructing the mockup, setting up video and other documentation,
and witnessing of the test. Since a fire test laboratory already has the HRR and
other instrumentation necessary, the marginal cost is small for setting up the
instrumentation and collecting the necessary data. The actual laboratory test
procedures  are, by now, quite well worked out, and time does not need to be
allocated to research in this area.
|Fire modeling can normally be considered as the prediction of fire
characteristics by the use of a mathematical method which is expressed as a
The needs of fire litigation from fire modeling are
specialized. Usually, there is a great deal of specificity about the sequence of
fire ignition and the materials involved in the process. This commonly precludes
the use of handbook data as input to fire models. Instead, it will usually be
necessary to conduct a sectional full-scale mockup to obtain appropriate data
describing the initial part of the fire. This information then serves as input
to a fire model, using which the later fire development can be approximately
| Babrauskas, V., COMPF: A Program for Calculating Post-flashover Fire
Temperatures (UCB FRG 75-2). Fire Research Group, University of California,
 Babrauskas, V., Fire Modeling Tools for Fire Safety Engineering: Are They
Good Enough? J. Fire Protection Engineering 8, 87-95 (1996).
 Babrauskas, V., and Peacock, R. D., Heat Release Rate: The Single Most
Important Variable in Fire Hazard, Fire Safety J. 18, 255-272
 Bukowski, R. W., Peacock, R. D., Jones, W. W., and Forney,
C. L., HAZARD I Fire Hazard Assessment Method (NIST Handbook 146). [U.S.] Natl.
Inst. Stand. Tech., Gaithersburg, MD.
 Babrauskas, V., Burning Rates (Section 3/Chapter 1), pp. 3-1 to 3-15 in
The SFPE Handbook of Fire Protection Engineering, Second Edition,
National Fire Protection Association, Quincy MA (1995).
 Babrauskas, V., Will the Second Item Ignite? Fire Safety J.
 Babrauskas, V., and Grayson, S. J.,
eds., Heat Release in Fires, E. & F. N. Spon, London (1992).
|This article Copyright © 1996, 1997 by Vytenis Babrauskas.